This invention relates to combination electrodes of the type which include both the measuring and reference electrodes of a potentiometric electrode system. The electrode combination may be a pH electrode, other specific ion electrodes or a redox electrode.
More particularly the invention relates to a structure for a combination electrode which minimizes the effects of harsh environments such as voltage gradients within a process sample.
In the past there has been considerable difficulty in getting a consistent pH measurement, for example, in plating baths and chlorine production cells because of the large voltage gradients which exists in such process samples. For example, in a chlorine production cell the bath may carry 80,000 amperes of d.c. current and it may be at 250 volt d.c. above ground. In such applications pH electrodes of all available types have been considered and many complicated arrangements have been attempted to avoid the effects on the pH measurement of the high voltage gradients occurring in the bath. Until the invention of the present combination electrode previous attempts to obtain consistent, dependable pH measurements in such baths have not been successful.
As is well known in electrochemical measurements, and specifically the potentiometric type measurements, there is usually utilized a measuring half cell and a reference half cell. The measuring half cell may be a high impedance device such as a glass electrode, for the determination of pH, that generates a potential with respect to the liquid sample medium such that that potential is a function of the hydrogen ion concentration of the sample. The reference electrode provides a potential that is substantially independent of the variable composition of the sample. The reference half cell comprises generally a piece of metal in contact with a mass of sparingly soluble salts of that metal, the assembly being placed in a salt bridge solution. The salt bridge solution has a nonmetallic ion in common with the sparingly soluble metal salt. In order to establish contact between the metal and the sample solution there is generally provided a reference junction that allows a constant diffusion of the salt bridge solution from the reference electrode when the electrode is wetted by the sample. Well known examples of reference half cells, commonly referred to as reference electrodes, are the silver-silver chloride electrodes and the calomel electrode. The electrolyte or salt bridge solution for both types is usually potassium chloride.
Many different structures have been utilized to provide the reference junction of the reference electrode. Such reference junction structures have included the use of thin ceramic coatings on the inner portion of the body of the reference electrode which provides a very limited leakage path between the reference electrode electrolyte and the sample. Such arrangements have been suggested where the centrally oriented glass electrode for pH measurement is surrounded by the body portion of the reference electrode whose end is lined with a ceramic coating with the space between that ceramic coating and body reference electrode being sealed by an elastomeric element.
Others have utilized elastomeric elements sealing the cavity between a central pH electrode and a surrounding electrode body where the body of the glass electrode has been roughened to provide for a small amount of leakage of the reference electrode electrolytes. In both of the above mentioned arrangements the leakage of the electrolyte takes place over a complete 360.degree. arcuate span and all of these prior devices have utilized liquid electrolytes which require, under normal conditions, frequent replenishment. These prior art devices all have the disadvantage of providing a reference junction whose resistivity is not uniform throughout the 360.degree. span over which it is effective and hence electrodes using such arrangements are subject to erroneous measurements when the electrodes are used in samples subject to high voltage gradients.
The arrangements of the prior art described above also generally have the disadvantage of being subject to clogging, particularly where the sample is likely to coat or foul the leaking region.
It is an object of the present invention to provide a combination measuring reference electrode which would be substantially free from the disadvantages of the prior art when utilized in samples subject to voltage gradients.